Microrheology and microstructure of water-in-water emulsions containing sodium caseinate and locust bean gum.

The mechanical response on the microscale of phase-separated water-in-water emulsions containing sodium caseinate (SCN) and locust bean gum (LBG) has been monitored by confocal laser scanning microscopy and particle tracking microrheology. Mixed biopolymer systems exhibiting phase-separated micro-regions were enriched in either protein or polysaccharide in the continuous or dispersed phase, depending on the weight ratio of the two biopolymers. Measurements of the tracking of charged probe particles revealed that the local rheological properties of protein-rich regions were considerably lower than that of LBG-rich domains for all the biopolymer ratios examined. At pH 7 in the absence of added salt, the viscosity of the protein-rich regions was little affected by an increase in overall LBG concentration, which is consistent with the phase separation mechanism in the mixed solution of charged (SCN) and uncharged (LBG) biopolymers being dominated by the relative entropy of the counter-ions associated with the charged protein molecules. Addition of salt was found to produce an enhancement in the level of thermodynamic incompatibility, leading to faster and more pronounced phase separation, and altering the micro-viscosity of protein-rich regions. At high ionic strength, it was also noted that there was a pronounced accumulation of incorporated probe particles at the liquid-liquid interface. The microrheological properties of the SCN-rich regions were found to be substantially pH-dependent in the range 7 > pH > 5.4. By adjusting the acidification conditions and the biopolymer ratio, discrete protein-based microspheres were generated with potential applications as a functional food ingredient.

Double Emulsions Relevant to Food Systems: Preparation, Stability, and Applications.

This review describes advances in the preparation of food-relevant double emulsions (DEs) of the water-in-oil-in-water (W/O/W) and oil-in-water-in-oil (O/W/O) types with emphasis on research published within the last decade. The information is assembled and critically evaluated according to the following aspects: the food application area, the range of encapsulated components and emulsion composition, the emulsification preparation methods, the balancing of the osmotic pressure, the stabilization by increased viscosity or gelation, the role of protein-polysaccharide interactions, and the techniques used to estimate DE yield and emulsification efficiency. Particular focus is directed toward the control of encapsulation and release behavior, including strategies that have been employed to improve the retention ability of the inner phase droplets by modifying the outer oil-water interface through mixed ingredient interactions, Pickering stabilization by particles, and biopolymer gelation. We also briefly consider the incorporation of DEs into dried microcapsules and the stability of W/O/W emulsions during eating and digestion. It would appear that 2 outstanding issues are currently preventing full realization of the potential of DEs in food applications: (i) the lack of availability of large-scale production equipment to ensure efficient nondestructive 2nd-stage emulsification, and (ii) the limited range of food-grade ingredients available to successfully replace polyglycerol polyricinoleate as the primary emulsifier in W/O/W formulations.

Structuring of colloidal particles at interfaces and the relationship to food emulsion and foam stability.

We consider the influence of spherical colloidal particles on the structure and stabilization of dispersions, emulsions and foams. Emphasis is placed on developments in the use of the methods of liquid state theory and computer simulation to understand short-range structuring of concentrated colloidal dispersions and ordering of particle layers near surfaces and within liquid films. Experimental information on the structuring of surfactant micelles and caseinate particles in thin liquid films is described, including an assessment of the effect of particle polydispersity on depletion interactions and kinetic structural stabilization. We specifically discuss the relevance of some of these structural concepts to the stability of food colloids.

Colloids in food: ingredients, structure, and stability.

This article reviews progress in the field of food colloids with particular emphasis on advances in novel functional ingredients and nanoscale structuring. Specific aspects of ingredient development described here are the stabilization of bubbles and foams by the protein hydrophobin, the emulsifying characteristics of Maillard-type protein-polysaccharide conjugates, the structural and functional properties of protein fibrils, and the Pickering stabilization of dispersed droplets by food-grade nanoparticles and microparticles. Building on advances in the nanoscience of biological materials, the application of structural design principles to the fabrication of edible colloids is leading to progress in the fabrication of functional dispersed systems-multilayer interfaces, multiple emulsions, and gel-like emulsions. The associated physicochemical insight is contributing to our mechanistic understanding of oral processing and textural perception of food systems and to the development of colloid-based strategies to control delivery of nutrients during food digestion within the human gastrointestinal tract.

First-order phase transition during displacement of amphiphilic biomacromolecules from interfaces by surfactant molecules.

The adsorption of surfactants onto a hydrophobic interface, already laden with a fixed number of amphiphilic macromolecules, is studied using the self consistent field calculation method of Scheutjens and Fleer. For biopolymers having unfavourable interactions with the surfactant molecules, the adsorption isotherms show an abrupt jump at a certain value of surfactant bulk concentration. Alternatively, the same behaviour is exhibited when the number of amphiphilic chains on the interface is decreased. We show that this sudden jump is associated with a first-order phase transition, by calculating the free energy values for the stable and the metastable states at both sides of the transition point. We also observe that the transition can occur for two approaching surfaces, from a high surfactant coverage phase to a low surfactant coverage one, at sufficiently close separation distances. The consequence of this finding for the steric colloidal interactions, induced by the overlap of two biopolymer + surfactant films, is explored. In particular, a significantly different interaction, in terms of its magnitude and range, is predicted for these two phases. We also consider the relevance of the current study to problems involving the competitive displacement of proteins by surfactants in food colloid systems.

Structure and rheology of colloidal particle gels: insight from computer simulation.

A particle gel is a network of aggregated colloidal particles with soft solid-like mechanical properties. Its structural and rheological properties, and the kinetics of its formation, are dependent on the sizes and shapes of the constituent particles, the volume fraction of the particles, and the nature of the interactions between the particles before, during and after gelation. Particle gels may be permanent or transient depending on whether the colloidal forces between the aggregating particles lead to irreversible bonding or weak reversible interactions. With short-range reversible interactions, network formation is typically associated with phase separation or kinetic arrest due to particle crowding. Much existing knowledge has been derived from computer simulations of idealized model systems containing spherical particles interacting with well-defined pair potentials. The status of current progress is reviewed here by summarizing the underlying methodology and key findings from a range of simulation approaches: Monte Carlo, molecular dynamics, Brownian dynamics, Stokesian dynamics, dissipative particle dynamics, multiparticle collision dynamics, and fluid particle dynamics. Then it is described how the technique of Brownian dynamics simulation, in particular, has provided detailed insight into how different kinds of bonding and weak reversible interactions can affect the aggregate fractal structure, the percolation behaviour, and the small-deformation rheological properties of network-forming colloidal systems. A significant ongoing development has been the establishment and testing of efficient algorithms that are able to capture the subtle dynamic structuring effects that arise from effects of interparticle hydrodynamic interactions. This has led to an appreciation recently of the potentially important role of these particle-particle hydrodynamic effects in controlling the evolving morphology of simulated colloidal aggregates and in defining the location of the sol-gel phase boundary.

Stabilising emulsion-based colloidal structures with mixed food ingredients.

The physical scientist views food as a complex form of soft matter. The complexity has its origin in the numerous ingredients that are typically mixed together and the subtle variations in microstructure and texture induced by thermal and mechanical processing. The colloid science approach to food product formulation is based on the assumption that the major product attributes such as appearance, rheology and physical stability are determined by the spatial distribution and interactions of a small number of generic structural entities (biopolymers, particles, droplets, bubbles, crystals) organised in various kinds of structural arrangements (layers, complexes, aggregates, networks). This review describes some recent advances in this field with reference to three discrete classes of dispersed systems: particle-stabilised emulsions, emulsion gels and aerated emulsions. Particular attention is directed towards explaining the crucial role of the macromolecular ingredients (proteins and polysaccharides) in controlling the formation and stabilisation of the colloidal structures. The ultimate objective of this research is to provide the basic physicochemical insight required for the reliable manufacture of novel structured foods with an appealing taste and texture, whilst incorporating a more healthy set of ingredients than those found in many existing traditional products.

Interfacial study of class II hydrophobin and its mixtures with milk proteins: relationship to bubble stability.

Class II hydrophobin (HFBII) is a very promising ingredient for improving food foam stability. Pure HFBII-stabilized bubbles exhibited exceptional stability to disproportionation (dissolution) but were not stable to bubble coalescence induced by a pressure drop. Bubbles stabilized by mixtures of HFBII + sodium caseinate (SC) or ß-lactoglobulin (BL) showed decreased shrinkage rates compared to pure SC or BL and improved the stability to pressure-drop-induced coalescence. Higher bubble stability was more closely correlated with higher surface shear viscosity than the surface dilatational elasticity of the mixed protein systems. Brewster angle microscopy observations and the high shear strength of adsorbed films, including HFBII, even in the presence of hydrophobic and hydrogen-bond-breaking agents, confirm that intermolecular attractive cross-links are unlikely to be the origin of the high strength of HFBII films. Possibly the HFBII molecules form a tightly interlocking monolayer of Janus-like particles at the air-water interface.

Simple statistical thermodynamic model of the heteroaggregation and gelation of dispersions and emulsions.

The heteroaggregation behaviour of mixtures of equal-sized particles (type A+type B) exhibiting short-ranged attractive interactions is investigated using the sticky hard-sphere model. The average cluster size is calculated as a function of the total particle volume fraction, the binary mixture composition, and the A-B stickiness interaction parameter τ(AB)(-1). We show that a value of τ(AB)(-1)=10(2), equivalent to an attractive well depth of ∼5kT in a realistic continuous pair potential, leads to a state of heteroaggregation just below the gelation threshold of the equimolar mixture of volume fraction 0.1. We discuss the conditions under which the assumptions of this statistical thermodynamic model are satisfied experimentally, with particular reference to recent data on the heteroaggregation behaviour of protein-stabilized emulsions and latex particle dispersions.

Food colloids research: historical perspective and outlook.

Trends and past achievements in the field of food colloids are reviewed. Specific mention is made of advances in knowledge and understanding in the areas of (i) structure and rheology of protein gels, (ii) properties of adsorbed protein layers, (iii) functionality derived from protein-polysaccharide interactions, and (iv) oral processing of food colloids. Amongst ongoing experimental developments, the technique of particle tracking for monitoring local dynamics and microrheology of food colloids is highlighted. The future outlook offers exciting challenges with expected continued growth in research into digestion processes, encapsulation, controlled delivery, and nanoscience.

Flocculation of protein-stabilized oil-in-water emulsions.

The flocculation properties of oil-in-water emulsions stabilized by proteins are reviewed from the colloid science perspective. Emphasis is placed on insight from systematic studies of the stability of emulsions prepared with a milk protein ingredient as the sole emulsifying agent. The main factors considered are pH, ionic strength, calcium ion concentration, thermal processing, and the presence of cosolutes (alcohol, sugars). Contrasting dependences of the flocculation behaviour on these factors are observed for the pH-sensitive disordered caseins (alpha(s1)-casein or beta-casein) and the heat-sensitive globular proteins (especially beta-lactoglobulin). In comparing characteristic emulsion properties obtained with different proteins, we consider the relative importance of the different kinds of molecular and colloidal interactions-electrostatic, steric, hydrophobic and covalent.

On the kinetics of acid sodium caseinate gelation using particle tracking to probe the microrheology.

The sol-gel transition of a model dairy system (sodium caseinate solution) which undergoes gelation by acidification has been studied by conventional bulk rheology and particle tracking microrheology, via confocal microscopy. The Brownian diffusion of fluorescent microspheres (0.21, 0.32, 0.5, and 0.89 µm in diameter) with different surface coatings (polyethylene glycol, carboxylate groups and polystyrene) was used to probe spatial mechanical properties of the gels at the scale of microns. The microrheological results are compared with the macroscopic viscoelastic properties (storage and loss shear modulus) measured in a concentric cylinder rheometer (double gap, at shear strain of 0.005 and frequency of 1 Hz). At pH values close to pI of the caseins, where formation of a protein network, i.e., gelation, became obvious from the confocal microscopy and bulk rheological measurements, all the particles had a tendency to adhere to the network. In spite of this, the microrheological values of the moduli were only slightly lower than the macroscopically determined values and the gel points calculated via both techniques tended to be in good agreement. However, the particle tracking method has higher sensitivity and can detect changes in the structuring of the system before these are registered by the bulk rheological measurement.

Mixed layers of sodium caseinate + dextran sulfate: influence of order of addition to oil-water interface.

We report on the interfacial properties of electrostatic complexes of protein (sodium caseinate) with a highly sulfated polysaccharide (dextran sulfate). Two routes were investigated for preparation of adsorbed layers at the n-tetradecane-water interface at pH = 6. Bilayers were made by the layer-by-layer deposition technique whereby polysaccharide was added to a previously established protein-stabilized interface. Mixed layers were made by the conventional one-step method in which soluble protein-polysaccharide complexes were adsorbed directly at the interface. Protein + polysaccharide systems gave a slower decay of interfacial tension and stronger dilatational viscoelastic properties than the protein alone, but there was no significant difference in dilatational properties between mixed layers and bilayers. Conversely, shear rheology experiments exhibited significant differences between the two kinds of interfacial layers, with the mixed system giving much stronger interfacial films than the bilayer system, i.e., shear viscosities and moduli at least an order of magnitude higher. The film shear viscoelasticity was further enhanced by acidification of the biopolymer mixture to pH = 2 prior to interface formation. Taken together, these measurements provide insight into the origin of previously reported differences in stability properties of oil-in-water emulsions made by the bilayer and mixed layer approaches. Addition of a proteolytic enzyme (trypsin) to both types of interfaces led to a significant increase in the elastic modulus of the film, suggesting that the enzyme was adsorbed at the interface via complexation with dextran sulfate. Overall, this study has confirmed the potential of shear rheology as a highly sensitive probe of associative electrostatic interactions and interfacial structure in mixed biopolymer layers.

Interfacial structuring in a phase-separating mixed biopolymer solution containing colloidal particles.

We report confocal microscopy observations of the spatial distribution of monodisperse charge-stabilized colloidal particles (amphoteric polystyrene latex) incorporated within a spinodal-type phase-separating system of mixed biopolymers (gelatin + oxidized starch). Images from samples aged at 40 degrees C demonstrate a strong tendency for the added particles to accumulate at the liquid-liquid interface and to influence the rate of coarsening of the complex bicontinuous microstructure. Large variations in the local curvature of particle-rich interfacial regions are suggestive of a liquid-liquid boundary that is substantially viscoelastic.

Mixed protein-polysaccharide interfacial layers: a self consistent field calculation study.

Mixed interfacial films of protein and polysaccharide have been investigated using self consistent field (SCF) calculations. The colloidal interactions mediated by such composite layers between two approaching surfaces have been considered. Two types of systems have been studied: (a) covalently-bonded polysaccharide and protein, and (b) the excess presence of polysaccharide at the interface, occurring through electrostatic interaction with an already existing, oppositely charged, protein layer. Our calculations show that for covalently-bonded complexes, depending on the location of the protein-polysaccharide bond, the attachment of short uncharged chains to the protein can be detrimental to provision of repulsive colloidal forces by such complexes. We have attributed this to an increased tendency of the hybrid polymer to adopt bridging configurations in the gap between two nearby surfaces. For larger grafted chains this bridging effect is eliminated, and the expected enhanced steric stabilization of the protein-polysaccharide conjugate is achieved. For adsorbed films formed through electrostatic interactions between these two biopolymers, stronger repulsive forces between the surfaces are produced, at an intermediate level of charging for the polysaccharides. This has been related to a maximum level of adsorption of polysaccharide, as the number of charged segments of the chain is varied. The peak occurs at higher levels of charging as the salt concentration in the bulk solution is increased. We have also observed the experimentally-reported phenomenon of charge overcompensation, arising from adsorption of the polysaccharide chains onto the primary protein layer. The importance of the non-uniform charge distribution of the polysaccharide molecule, in providing an explanation for this effect, has been demonstrated.

Interactions between adsorbed layers of alphaS1-casein with covalently bound side chains: a self-consistent field study.

The effect on the adsorbed layer properties of the modification of alpha S1-casein by covalent bonding with an uncharged polysaccharide side chain has been investigated using lattice-based self-consistent field (SCF) theory. Interactions between two hydrophobic planar surfaces coated by a layer of adsorbed modified alpha S1-casein have been studied as a function of pH and ionic strength. While the interactions of the unmodified alpha S1-casein layers become attractive at high ionic strength, it has been shown that the presence of polysaccharide attachment to the alpha S1-casein molecule can confer net repulsive interactions over a wide range of salt concentration. The disordered protein is represented as a linear flexible polyampholyte with a sequence of hydrophobic, polar, and charged units based on the known alpha S1-casein primary structure. The hydrophilic side chain is attached at various fixed positions along the casein backbone. Different lengths and locations of the attached polysaccharide side chain are examined. Interfacial structures and colloidal stability properties of the system are determined, including the surface-surface interaction potential, the extent of protein bridging, and the distribution of protein segments from the surface under different conditions of pH and ionic strength. It has been found that the covalent bonding of short hydrophilic chains may not only enhance but can also worsen the colloidal stabilizing properties of the modified protein, depending on the position of the attachment.

Morphological changes in adsorbed protein films at the oil-water interface subjected to compression, expansion, and heat processing.

Adsorbed films of milk proteins at the oil-water (O-W) interface have been imaged using a Brewster angle microscope (BAM). Special adaptations were made to the BAM to allow imaging of the O-W interface and to enable in situ heating and cooling of the adsorbed films. The proteins beta-lactoglobulin (beta-L) and alphas1-, beta-, and kappa-casein were studied over a range of bulk protein concentrations (Cb) and surface ages at pH 7 and for beta-L at pH 5 also. The adsorbed films were subjected to incremental compression and expansion cycles, such that the film area was typically varied between 125% and 50% of the original film area, and the resulting film structure was recorded via the BAM at 25.0 degrees C. Structuring of beta-L films (the formation of ridges and cracks) was more pronounced at pH 5 (closer to the protein's isoelectric point) than at pH 7 and for longer adsorption times and/or higher Cb. Structuring was also much more apparent at the O-W interface than at the A-W interface on compression/expansion/aging, especially at pH 7. After heating beta-L films adsorbed at low Cb (0.005 wt %) to 80 or 90 degrees C, an even greater degree of film structuring was evident, but beta-L films adsorbed at higher Cb (> or =0.05 wt %) showed fewer but larger fractures. The adsorbed caseins showed little evidence of such features, either before or after heating, apart from slight structuring for the heated films of alphas1- and kappa-casein films after 1 day. Changes in the dilatational elastic modulus of the beta-L films (Cb = 0.005 wt %) were correlated with the variations in the structural integrity of the films as observed via the BAM technique. In particular, there was a marked increase in the elastic modulus on heating, while the cycle of compression and expansion appeared to result in a net film weakening overall. The beta-L films adsorbed at higher Cb (> or =0.05 wt %) behaved as if an even stronger elastic skin completely covered the interface. The overall conclusion is that interfacial protein films subjected to these types of thermal and mechanical perturbations, which are typical of those that occur in food colloid processing, can become highly inhomogeneous, depending on the type of protein and the bulk solution conditions. This undoubtedly has implications for the stability of the corresponding emulsions and foams.